1. Field of the Invention
This invention relates to a noble metal plated material applied with a noble metal plating and employed for parts of electronic and electric equipment, such as lead frames and terminal connectors, and to a strike plating solution useful for the formation of a primer plating of the plated material.
2. Description of the Prior Art
Plated materials, each of which is composed of a metal plate such as copper alloy or stainless steel plate and a noble metal plating of gold(Au), silver(Ag) or the like applied thereon, have been used widely for parts of various electronic and electric equipment owing to their excellent chemical and physical properties. Plated materials applied with such noble metal platings as mentioned above are however very expensive due to the high prices of the noble metals. It is hence desired to make their plating layers as thin as possible. On the other hand, noble metal plated materials as parts of electronic and electric equipment may be exposed to heat as high as several hundred degrees. In such a high-temperature atmosphere, the noble metal plated materials involve such problems that their solderability, bonderability and the like may be deteriorated considerably and tarnishing may also take place due to oxidation of the basis metals of the plated materials, diffusion of the basis metals into their corresponding noble metal plating layers, etc.
With a view toward avoiding the above-mentioned quality deterioration of a noble metal plated material in such a high-temperature atmosphere, it is now practiced to apply a primer plating before application of a noble metal plating to a basis metal as a material to be plated. For example, it has already been attempted to plate Sn--Ni, Sn--Co, Ni or the like as a primer plating on stainless steel as a metal to be plated. Namely, by using a plating solution which contains NiCl.sub.2 and HCl and is a nickel strike plating solution called "Wood's bath", stainless steel as a material to be plated is plated for about 2 -4 minutes at a current density of 2-20 A/dm.sup.2 so as to apply a primer plating in the form of a nickel plating layer of about 1 .mu.m thick, followed by further plating of a noble metal such as Au or Ag on the primer plating.
Even if such a primer plating is applied, it is still impossible to obtain a noble metal plated material capable of exhibiting sufficient solderability and bonderability after being held for about 3 minutes in an air atmosphere of 450.degree. C. so long as its noble metal plating layer is for example an Au or Ag plating as thin as 0.1 .mu.m or so. It is therefore indispensable to apply a noble metal plating of a significant thickness on a primer plating in order to obtain sufficient solderability and bonderability. The aforementioned problem that the high costs of noble metal plated materials are unavoidable still remains accordingly.
It may be contemplated to use a copper strike plating solution, which is routinely employed to form a film having good adhesion to a basis metal such as steel or a diecasting zinc alloy, for the formation of a primer plating for a noble metal plated material, since the copper strike plating solution can also improve the bonding properties of noble metal platings such as gold platings and silver platings. A noble metal plated material, which has been obtained by applying a primer plating in the copper strike plating solution and then forming a noble metal plating such as gold or silver plating on the primer plating, however shows a sign of reduced heat resistance that its solderability and bonderability (wire bonderability) are lowered to a significant extent, when it is heated in the atmosphere. Use of such noble metal plated materials for parts of electronic and electric equipment such as lead frames and terminal connectors, has hence been accompanied by a drawback that the reliability is poor.
It has thus been attempted to use a cobalt-nickel alloy plating as the above-mentioned primer plating in order to avoid the deterioration of heat resistance.
Plating baths for such cobalt-nickel platings generally include those making use of sulfates, chlorides, sulfate/chloride mixtures, sulfamates, borofluorates, pyrophosphates, etc. These plating baths are employed for plating magnetic materials, molds for plastics and hard press dies, ornaments and accessories, copper alloys and stainless steel, etc.
Such a cobalt-nickel alloy plating is conducted at a high cathode efficiency, usually, of 30% or higher in view of productivity and cost. It has hence been extremely difficult to plate a dense and thin film evenly. The solderability and bonderability of plated materials applied with cobalt-nickel alloy platings has not been improved as much as they were expected. Moreover, there is another drawback of inefficiency in plating work that nickel strike plating must be applied in advance before applying a cobalt-nickel alloy plating on stainless steel. Cobalt-nickel alloy platings are therefore not satisfactory as primer platings for noble metal plated materials which are employed as lead frames, terminal connectors and the like.